Spin-on filters have been employed heretofore in a variety of applications including hydraulic systems and engine lubrication systems. Such filters generally include a filter element within a can or housing having a cover or attachment plate at one end thereof by which the filter can be screwed onto or off of a filter head. A central opening and several surrounding openings in the cover direct fluid flow through the filter, which flow can be in either an inside/out or outside/in direction relative to the filter element. A circular gasket on the outside of the cover serves as the external seal between the filter and the filter head, while another circular gasket on the inside of the cover functions as the internal seal between the filter element and cover. A spring is often provided in the lower end of the housing to maintain the filter element in sealing engagement with the cover. Spin-on filters are usually intended to be used only once before removal and replacement.
Although satisfactory in low- and medium-pressure applications, most spin-on filters of the prior art have not been particularly suitable for use in high-pressure applications, such as in hydraulic transmission pumps, where spikes or surges up to about 1,000 psi can occur. Many of the spin-on filters currently available are adaptations of the type used in engine lubrication systems. The covers of such spin-on filters are typically constructed of a stamped steel-based disc and a relatively thinner secondary disc spot welded thereto. The base disc includes an extruded, relatively shallow, internally threaded neck portion by which the filter can be connected to a filter head. Flow openings are punched into the base disc around the neck portion. The lip at the open end of the housing is connected by means of a lock seam to the periphery of the secondary disc, which is also formed to serve as a seat for the external gasket. In spin-on filters of this type, any fatigue failure is most likely to occur at the rolled lock seam or at the spot welds. Any burst failure is most likely to occur either upon bending of the cover, which allows leakage past the external gasket, or upon unfolding of the rolled lock seam. The prior spin-on filters of this type have thus been susceptible to failure at the cover and/or at the connection between the cover and the housing.
Various attempts have been made to strengthen and otherwise increase the pressure capacities of the prior spin-on filters. Different materials and/or increased material thicknesses have been used, improved lock seams have been developed, and reinforcing profiles have been formed into the cover plates. These efforts have resulted in increasing the burst capacities of such spin-on filters up to about 500 psi, and have therefore been of some success; however, even filters of such capacity can be marginal in certain applications. In addition, reinforcing efforts of this type tend to increase the cost of such filters. It will be understood that manufacturing limitations and production economies can be important factors in the construction of such spin-on filters.
More recently, U.S. Pat. No. 4,369,113 issued to Donaldson Company, Inc., for an improved high-strength spin-on filter which overcomes many of the disadvantages of the prior art. This spin-on filter is capable of withstanding pressure surges and spikes up to about 1,000 psi or more, and has met with considerable commercial success. While suitable for use in many high-pressure applications, however, it has been found that fatigue strength can be just as important as pressure capacity in certain applications, such as hydrostatic transmissions and charge pump circuits, involving cyclical operational loads.
There is thus a need for an improved reinforced spin-on filter of high pressure capacity and better fatigue rating.